1) Preheat oven to 240C/Gas 9.
2) Place a tea-kettle of water on to boil for a water bath. With a serrated knife, remove the side crusts from the half piece of the panettone. Cut into 2.5cm dice. Reserve the cubed panettone in a large mixing bowl.
3) For the eggnog custard, in another bowl thoroughly whisk together the eggs, yolks, sugar, half cream, rum, vanilla essence, and a healthy grating of fresh nutmeg. Pour this over the bread cubes.
4) Spray a 12 hole muffin tin with vegetable cooking spray. Ladle the bread/eggnog mixture gently and evenly into the muffin tins. The big cubes sticking up look nice. Place the filled muffin tin in a tall sided roasting pan. Transfer to the preheated oven and carefully pour the hot water from kettle onto sheet pan, creating a water bath for the muffin tin to sit in.
5) Bake 15 to 20 minutes until the tops are nicely browned, and a toothpick comes clean from the center.
6) Bread pudding may be served warm or cold. Vanilla Ice cream or whipped cream would be great accompaniments.
7) Alternately place the muffin tin in a roasting pan on the stove. Pour in up to 1-inch of boiling water. Cover and cook over medium-high heat for 15 to 20 minutes. The puddings will puff and a toothpick will come out clean from the center. The tops won't brown with this method. Perhaps a sprinkle of cinnamon sugar at serving time would be nice.